CyberKnife®
is a variant of photon EBRT (External Beam Radiation Treatment) but like HDR
(High Dosage Radiation) Brachytherapy, it is based on delivering a Hypo-fractionated
Radiation Dosage -the theory being that the prostate gland and cancerous tumors
associated with it can be treated with a lower total dose of radiation but with
higher individual doses
(hypofractionated doses)
of radiation and fewer treatments without increasing damage to surrounding organs
and tissue.
The procedure uses sophisticated equipment with a good record
in certain tumors (e.g. nervous system), but is, at this date (February 2008)
regarded as 'experimental' for prostate cancer because there are very few studies
and no long term studies as to its efficacy and morbidity results.
Its
potential benefits are
(a) sophisticated tracking of prostate position
- it is claimed that the stereotatic heads can track very small movements of the
prostate and deliver doses more accurately than other methods
This sophisticated,
unique method of tracking prostate location may be particularly necessary because
the patient has to lie for an hour or so for each CyberKnife® session. This
contrasts, for example, with a session of conformal proton
beam as currently practiced, where the sessions are a minute or so, hardly
giving the prostate a chance to move, with an exception that applies to both cases:
Respiratory motion which has been reported to cause an uncertainty of about 3
mm in prostate location. It seems unlikely that the sampling of location with
CyberKnife is so frequent and the response time so short that there is a correction
for respiratory motion. There is also the question as to how much radiation the
multiple position-tracking x-rays deliver to healthy tissue.
(b) the
somewhat hypothetical benefits of rather extreme hypofractionation, which means
that there are many fewer sessions of treatment (certainly convenient), with a
higher than conventional dose per session, BUT a LOWER total dose of radiation
to the prostate over the course. The theory is that such a scheme may offer better
results, because it creates a greater differential between the prostate and normal
tissue, i.e. is more brutal to the cancer and kinder to the normal tissue that
is irradiated. There is considerable disagreement among scientists and radiation
oncologists about the efficacy and long-term side effects of this procedure.
This
study (in pdf format) gives some technical background to the concept but it
should be noted that the data supporting the theory is somewhat limited, being
gathered from one person who may have been very carefully chosen. The paper suggests
that the basic technique can produce a tighter zone of high radiation around the
prostate than IMRT
(Intensity Modulated Radiation Therapy), with better sparing of the rectum
and bladder, Assuming that the high radiation zone really is tight, then it may
be necessary to consider that there is a trade-off: a tight zone might spare adjacent
organs but also fail to radiate areas of close-in local spread of cancer.
There
is some evidence to support the idea of hypofractionation, but overall it cannot
be denied that the procedure is experimental. Hypofractionation may well have
merit, but long term (>10 year) results are not available for HDR
Brachytherapy, which has been practised since about 1997, and neither long
nor short term (>5 year) peer reviewed results are yet available for CyberKnife®.
Maybe in the long run, CyberKnife® will prove to have similar (but possibly
not better) outcomes vis-a-vis non-hypofractionated EBRT (External Beam Radiation
Treatment), in which case, its selling point will be convenience.
Any man
considering Cyberknife® should make sure that his is a truly informed consent,
and that he has chosen it over methods with established records.
There
is an excellent on-line forum
that enables patients to get answers to questions from experts and share their
own experiences.
Another source for comparison of various prostate cancer
treatments, including IMRT, brachytherapy, HDR brachytherapy and CyberKnife was
found here at
CyberKnife® Center of San Diego
It is, however, a "sales pitch" for the Center, and should be considered in that
light.
